pyfakefs implements a fake file system that mocks the Python file system modules. Using pyfakefs, your tests operate on a fake file system in memory without touching the real disk. The software under test requires no modification to work with pyfakefs.

pyfakefs acts as a pytest plugin by providing the fs fixture, which will automatically invoke the fake filesystem. It also provides the fake_filesystem_unittest.TestCase class for use with unittest and the means to use the fake filesystem with other test frameworks.

pyfakefs works with current versions of Linux, Windows and macOS.

This document provides a general overview for pyfakefs. There is more:

• The documentation at Read the Docs:
  • The Release documentation contains usage documentation for pyfakefs and a description of the most relevant classes, methods and functions for the last version released on PyPI
  • The Development documentation contains the same documentation for the current main branch
  • The Release 3.7 documentation contains usage documentation for the last version of pyfakefs supporting Python 2.7
• The Release Notes show a list of changes in the latest versions

The simplest method to use pyfakefs is using the fs fixture with pytest. Refer to the usage documentation for information on other test scenarios, test customization and using convenience functions.

Apart from automatically mocking most file-system functions, pyfakefs provides some additional features:

• mapping files and directories from the real file system into the fake filesystem
• configuration and tracking of the file system size
• pause and resume of patching to be able to use the real file system inside a test step
• (limited) emulation of other OSes (Linux, macOS or Windows)
• configuration to behave as if running as a non-root user while running under root

pyfakefs works with CPython 3.7 and above, on Linux, Windows and macOS, and with PyPy3.

pyfakefs works with pytest version 3.0.0 or above, though a current version is recommended.

pyfakefs will not work with Python libraries that use C libraries to access the file system. This is because pyfakefs cannot patch the underlying C libraries' file access functions--the C libraries will always access the real file system. Refer to the documentation for more information about the limitations of pyfakefs.

pyfakefs is currently automatically tested on Linux, macOS and Windows, with Python 3.7 to 3.11, and with PyPy3 on Linux, using GitHub Actions.

pyfakefs unit tests can be run using pytest (all tests) or unittest (all tests except pytest-specific ones):

$ cd pyfakefs/
$ export PYTHONPATH=$PWD

$ python -m pytest pyfakefs
$ python -m pyfakefs.tests.all_tests

Similar scripts are called by tox and Github Actions. tox can be used to run tests locally against supported python versions:

$ tox

The Dockerfile at the repository root will run the tests on the latest Ubuntu version. Build the container:

cd pyfakefs/
docker build -t pyfakefs .

Run the unit tests in the container:

docker run -t pyfakefs

We always welcome contributions to the library. Check out the Contributing Guide for more information.

pyfakefs.py was initially developed at Google by Mike Bland as a modest fake implementation of core Python modules. It was introduced to all of Google in September 2006. Since then, it has been enhanced to extend its functionality and usefulness. At last count, pyfakefs was used in over 2,000 Python tests at Google.

Google released pyfakefs to the public in 2011 as Google Code project pyfakefs:

• Fork jmcgeheeiv-pyfakefs added direct support for unittest and doctest
• Fork shiffdane-jmcgeheeiv-pyfakefs added further corrections

After the shutdown of Google Code was announced, John McGehee merged all three Google Code projects together here on GitHub where an enthusiastic community actively supports, maintains and extends pyfakefs. In 2022, the repository has been transferred to pytest-dev to ensure continuous maintenance.